This invention relates generally to the field of monitoring of analytes in the body and the transdermal delivery of drugs to the body. More particularly, this invention relates to a minimally invasive to non-invasive method of increasing the permeability of the skin through microporation of the stratum corneum, which can be combined with sonic energy, chemical permeation enhancers, pressure, and the like for selectively enhancing outward flux of analytes from the body for monitoring thereof or the delivery of drugs into the body.
The stratum corneum is chiefly responsible for the well known barrier properties of skin. Thus, it is this layer that presents the greatest barrier to transdermal flux of drugs or other molecules into the body and of analytes out of the body. The stratum corneum, the outer horny layer of the skin, is a complex structure of compact keratinized cell remnants separated by lipid domains. Compared to the oral or gastric mucosa, the stratum corneum is much less permeable to molecules either external or internal to the body. The stratum corneum is formed from keratinocytes, which comprise the majority of epidermal cells, that lose their nuclei and become corneocytes. These dead cells comprise the stratum corneum, which has a thickness of only about 10-30 .mu.m and, as noted above, is a very resistant waterproof membrane that protects the body from invasion by exterior substances and the outward migration of fluids and dissolved molecules. The stratum corneum is continuously renewed by shedding of corneum cells during desquamination and the formation of new corneum cells by the keratinization process.
The flux of a drug or analyte across the skin can be increased by changing either the resistance (the diffusion coefficient) or the driving force (the gradient for diffusion). Flux may be enhanced by the use of so-called penetration or chemical enhancers. Chemical enhancers are well known in the art and a more detailed description will follow.
Another method of increasing the permeability of skin to drugs is iontophoresis. Iontophoresis involves the application of an external electric field and topical delivery of an ionized form of drug or an un-ionized drug carried with the water flux associated with ion transport (electro-osmosis). While permeation enhancement with iontophoresis has been effective, control of drug delivery and irreversible skin damage are problems associated with the technique.
Sonic energy has also been used to enhance permeability of the skin and synthetic membranes to drugs and other molecules. Ultrasound has been defined as mechanical pressure waves with frequencies above 20 kHz, H. Lutz et al., Manual of Ultrasound 3-12 (1984). Sonic energy is generated by vibrating a piezoelectric crystal or other electromechanical element by passing an alternating current through the material, R. Brucks et al., 6 Pharm. Res. 697 (1989). The use of sonic energy to increase the permeability of the skin to drug molecules has been termed sonophoresis or phonophoresis.
Although it has been acknowledged that enhancing permeability of the skin should theoretically make it possible to transport molecules from inside the body through the skin to outside the body for collection or monitoring, practicable methods have not been disclosed. U.S. Pat. No. 5,139,023 to Stanley et al. discloses an apparatus and method for noninvasive blood glucose monitoring. In this invention, chemical permeation enhancers are used to increase the permeability of mucosal tissue or skin to glucose. Glucose then passively diffuses through the mucosal tissue or skin and is captured in a receiving medium. The amount of glucose in the receiving medium is measured and correlated to determine the blood glucose level. However, as taught in Stanley et al., this method is much more efficient when used on mucosal tissue, such as buccal tissue, which results in detectable amounts of glucose being collected in the receiving medium after a lag time of about 10-20 minutes. However, the method taught by Stanley et al. results in an extremely long lag time, ranging from 2 to 24 hours depending on the chemical enhancer composition used, before detectable amounts of glucose can be detected diffusing through human skin (heat-separated epidermis) in vitro. These long lag times may be attributed to the length of time required for the chemical permeation enhancers to passively diffuse through the skin and to enhance the permeability of the barrier stratum corneum, as well as the length of time required for the glucose to passively diffuse out through the skin. Thus, Stanley et al. clearly does not teach a method for transporting blood glucose or other analytes non-invasively through the skin in a manner that allows for rapid monitoring, as is required for blood glucose monitoring of diabetic patients and for many other body analytes such blood electrolytes.
While the use of sonic energy for drug delivery is known, results have been largely disappointing in that enhancement of permeability has been relatively low. There is no consensus on the efficacy of sonic energy for increasing drug flux across the skin. While some studies report the success of sonophoresis, J. Davick et al., 68 Phys. Ther. 1672 (1988); J. Griffin et al., 47 Phys. Ther. 594 (1967); J. Griffin & J. Touchstone, 42 Am. J. Phys. Med. 77 (1963); J. Griffin et al., 44 Am. J. Phys. Med. 20 (1965); D. Levy et al., 83 J. Clin. Invest. 2074); D. Bommannan et al., 9 Pharm. Res. 559 (1992), others have obtained negative results, H. Benson et al., 69 Phys. Ther. 113 (1988); J. McElnay et al., 20 Br. J. Clin. Pharmacol. 4221 (1985); H. Pratzel et al., 13 J. Rheumatol. 1122 (1986). Systems in which rodent skin were employed showed the most promising results, whereas systems in which human skin was employed have generally shown disappointing results. It is well known to those skilled in the art that rodent skin is much more permeable than human skin, and consequently the above results do not teach one skilled in the art how to effectively utilize sonophoresis as applied to transdermal delivery and/or monitoring through human skin.
A significant improvement in the use of ultrasonic energy in the monitoring of analytes and also in the delivery of drugs to the body is disclosed and claimed in copending applications Ser. No. 08/152,442 filed Nov. 15, 1993, now U.S. Pat. No. 5,458,140, and Ser. No. 08/152,174 filed Dec. 8, 1993, now U.S. Pat. No. 5,445,611, both of which are incorporated herein by reference. In these inventions, the transdermal sampling of an analyte or the transdermal delivery of drugs, is accomplished through the use of sonic energy that is modulated in intensity, phase, or frequency or a combination of these parameters coupled with the use of chemical permeation enhancers. Also disclosed is the use of sonic energy, optionally with modulations of frequency, intensity, and/or phase, to controllably push and/or pump molecules through the stratum corneum via perforations introduced by needle puncture, hydraulic jet, laser, electroporation, or other methods.
The formation of micropores (i.e. microporation) in the stratum corneum to enhance the delivery of drugs has been the subject of various studies and has resulted in the issuance of patents for such techniques.
Jacques et al., 88 J. Invest. Dermatol. 88-93 (1987), teaches a method of administering a drug by ablating the stratum corneum of a region of the skin using pulsed laser light of wavelength, pulse length, pulse energy, pulse number, and pulse repetition rate sufficient to ablate the stratum corneum without significantly damaging the underlying epidermis and then applying the drug to the region of ablation. This work resulted in the issuance of U.S. Pat. No. 4,775,361 to Jacques et al. The ablation of skin through the use of ultraviolet-laser irradiation was earlier reported by Lane et al., 121 Arch. Dermatol. 609-617 (1985). Jacques et al. is restricted to use of few wavelengths of light and expensive lasers.
Tankovich, U.S. Pat. No. 5,165,418 (hereinafter, "Tankovich '418"), discloses a method of obtaining a blood sample by irradiating human or animal skin with one or more laser pulses of sufficient energy to cause the vaporization of skin tissue so as to produce a hole in the skin extending through the epidermis and to sever at least one blood vessel, causing a quantity of blood to be expelled through the hole such that it can be collected. Tankovich '418 thus is inadequate for noninvasive or minimally invasive permeabilization of the stratum corneum such that a drug call be delivered to the body or an analyte from the body can be analyzed.
Tankovich et al., U.S. Pat. No. 5,423,803 (hereinafter, "Tankovich '803") discloses a method of laser removal of superficial epidermal skin cells in human skin for cosmetic applications. The method comprises applying a light-absorbing "contaminant" to the outer layers of the epidermis and forcing some of this contaminant into the intercellular spaces in the stratum corneum, and illuminating the infiltrated skin with pulses of laser light of sufficient intensity that the amount of energy absorbed by the contaminant will cause the contaminant to explode with sufficient energy to tear off some of the epidermal skin cells. Tankovich '803 further teaches that there should be high absorption of energy by the contaminant at the wavelength of the laser beam, that the laser beam must be a pulsed beam of less than 1 .mu.S duration, that the contaminant must be forced into the upper layers of the epidermis, and that the contaminant must explode with sufficient energy to tear off epidermal cells upon absorption of the laser energy. This invention also fails to disclose or suggest a method of drug delivery or analyte collection.
Raven et al., WO 92/00106, describes a method of selectively removing unhealthy tissue from a body by administering to a selected tissue a compound that is highly absorbent of infrared radiation of wavelength 750-860 nm and irradiating the region with corresponding infrared radiation at a power sufficient to cause thermal vaporization of the tissue to which the compound was administered but insufficient to cause vaporization of tissue to which the compound had not been administered. The absorbent compound should be soluble in water or serum, such as indocyanine green, chlorophyll, porphyrins, heme-containing compounds, or compounds containing a polyene structure, and power levels are in the range of 50-1000 W/cm.sup.2 or even higher.
Konig et al., DD 259351, teaches a process for thermal treatment of tumor tissue that comprises depositing a medium in the tumor tissue that absorbs radiation in the red and/or near red infrared spectral region, and irradiating the infiltrated tissue with an appropriate wavelength of laser light. Absorbing media can include methylene blue, reduced porphyrin or its aggregates, and phthalocyanine blue. Methylene blue, which strongly absorbs at 600-700 nm, and a krypton laser emitting at 647 and 676 nm are exemplified. The power level should be at least 200 mW/cm.sup.2.
It has been shown that by stripping the stratum corneum from a small area of the skin with repeated application and removal of cellophane tape to the same location one can easily collect arbitrary quantities of interstitial fluid, which can then be assayed for a number of analytes of interest. Similarly, the `tape-stripped` skin has also been shown to be permeable to the transdermal delivery of compounds into the body. Unfortunately, `tape-stripping` leaves a open sore which takes weeks to heal, and for this, as well as other reasons, is not considered as an acceptable practice for enhancing transcutaneous transport in wide applications.
As discussed above, it has been shown that pulsed lasers, such as the excimer laser operating at 193 nm, the erbium laser operating near 2.9 .mu.m or the CO.sub.2 laser operating at 10.2 .mu.m, can be used to effectively ablate small holes in the human stratum corneum. These laser ablation techniques offer the potential for a selective and potentially non-traumatic method for opening a delivery and/or sampling hole through the stratum corneum. However, due to the prohibitively high costs associated with these light sources, there have been no commercial products developed based on this concept. The presently disclosed invention, by defining a method for directly conducting thermal energy into the stratum corneum with very tightly defined spatial and temporal resolution, makes it possible to produce the desired micro-ablation of the stratum corneum using very low cost energy sources.
In view of the foregoing problems and/or deficiencies, the development of a method for safely enhancing the permeability of the skin for minimally invasive or noninvasive monitoring of body analytes in a more rapid time frame would be a significant advancement in the art. It would be another significant advancement in the art to provide a method of minimally invasively or non-invasively enhancing the transdermal flux rate of a drug into a selected area of an individual's body.